Chromatography is a chemical method to find out the type and/or the amount of chemical substances in a gas or a solution.
The main principle of chromatography is that the gas/solution slowly spreads over a "stationary phase" (for example a piece of paper). Since different substances travel with different speed, the compounds of the gas/solution split up while spreading through the stationary phase and can be measured seperately.

Guy: "Hey, what kind of stuff is in those gas cylinders?"
Scientist: "Let's do a chromatography to find out".